The present invention relates to a fluid active suspension apparatus using compressible fluid as an operating medium and to an operation control method for the suspension apparatus.
In order to suppress vibrations of a vehicle body caused by vibration input from the road surface, it is known in the art to employ an active suspension apparatus in which positive damping is effected by supplying and discharging compressible fluid to and from fluid spring chambers. U.S. Pat. No. 4,856,815, for example, discloses a pneumatic suspension apparatus which is designed to keep the vehicle body horizontally positioned even when the vehicle is traveling on a wavy road. In this suspension apparatus, air is supplied to and discharged from air spring chambers of suspension units arranged between the vehicle body and respective wheels, in accordance with the output of a stroke sensor representing the relative displacement between the vehicle body and the wheels (more generally, between sprung and unsprung sides of the vehicle), to thereby control the stroke of the suspension units so that the sprung side can be held horizontally even when the unsprung side is making a vertical (up-and-down) motion in response to change in the level of the road surface.
More specifically, in this suspension apparatus, when the stroke sensor output becomes greater than or equal to a predetermined value, a supply valve is opened for a predetermined time to actively supply air to the air spring chamber if the air spring is expanding, and a discharge valve is opened for a predetermined time to actively discharge air from the air spring chamber if the air spring is contracting.
This type of suspension apparatuses can suppress vertical motion of the vehicle body, but is not sufficiently effective in reducing the force causing the vertical motion of the vehicle body itself. Thus, when sprung resonance conditions are fulfilled, in particular, bouncing vibration, which gives a strange feeling to a person in the vehicle, occurs in a sprung resonance region.
As a control technique for reducing such bouncing vibration, the skyhook damper theory is known in the art. According to this theory, a force proportional to the vertical velocity of the vehicle body is produced and is acted in a direction to cancel out the vertical velocity, to thereby suppress vibration of the vehicle body caused by the vibration transmitted from the road surface. So far, the skyhook damper theory is embodied in an active suspension apparatus using incompressible fluid as an operating medium.
For example, U.S. Pat. No. 5,089,966 discloses a hydraulic active suspension apparatus using oil as the operating medium. In this suspension apparatus, the vertical velocity of the vehicle body is obtained by detecting the vertical acceleration of the vehicle body and integrating the detected acceleration, and an oil pressure proportional to the vertical velocity is supplied to actuators, to thereby damp vibration of the vehicle body.
This type of hydraulic active suspension apparatuses comprises, as its indispensable elements, a hydraulic pressure source including a pump, a reservoir tank, and a pump accumulator for eliminating the pumping pulsation, a hydraulic control system including an actuator, a proportional control valve for applying a desired pressure of oil from the hydraulic pressure source to the actuator, and a main accumulator disposed between the proportional control valve and the actuator for accumulating the hydraulic pressure, and a controller for controlling the hydraulic control system.
This hydraulic active suspension apparatus is effective in carrying out damping based on the skyhook damper theory, but it requires a proportional control valve having high operation accuracy and high operation response, which leads to an increase of the cost. Further, since oil is used as the operating fluid, the hydraulic pressure source is large in size, requiring much space for the suspension apparatus and increasing the weight of the vehicle.
In the pneumatic active suspension apparatus, on the other hand, since air serving as the operating fluid has compressibility, it is difficult to control the internal pressure of the air spring chamber of the apparatus by using, e.g., a proportional control valve. Further, the pneumatic suspension apparatus is low in operation response, and it is, therefore, difficult to operate an actuator such as the air spring chamber at a proper time so as to produce a force for suppressing vibration of the vehicle body. For this reason, no skyhook damper theory-based control technique applicable to the pneumatic active suspension apparatus has been developed yet.
The aforementioned hydraulic active suspension apparatus capable of skyhook damper theory-based active control uses three or four sensors for detecting the vertical acceleration of the vehicle body, in order to acquire control information for use in the active control. The vertical acceleration sensors are, however, expensive, and an increase in the number of sensors generally requires more thoughtful consideration on the installation of sensors, correction of the sensor output characteristics in relation to temperature, configuration of a fail-safe system for troubleshooting the sensors, etc. Accordingly, the conventional apparatus using three or four vertical acceleration sensors is costly.